(a) Field of the invention
The present invention pertains to a power supply changeover type power amplifier for audio instruments, which is capable of materializing both an expansion of dynamic range and an improvement of power efficiency, without causing overload or other harms in the power supply unit and other constituting parts.
(b) Description of the prior art
For dynamic ranges in audio power amplifiers intended for hi-fi reproduction, there have been required enhanced values for faithful amplification of audio signals having wide dynamic range in amplitude such as music signals in which large amplitudes are occasionally contained in ordinary amplitudes. In case it is intended to obtain such wide dynamic ranges in the amplifiers, it is necessary to provide a power supply capable of outputting power supply voltages having large absolute values, i.e. high voltage power supply, in power amplifiers of said type, in order to faithfully amplify musical signals of such possible large amplitudes as mentioned above, i.e. in order not to cause clipping of the output signals. On the other hand, however, for a dynamic range sufficient to amplify musical signals having only ordinary amplitudes, it is better for a power amplifier of this type to be applied with a power supply designed for outputting power supply voltages having smaller absolute values, i.e. low voltage power supply, from the viewpoint that, for example, the collector dissipation of the constituent output transistor is reduced, and that the power efficiency is thus enhanced. And, moreover, in view of the fact that, musical signals which are amplified by an ordinary audio power amplifier very seldom contain trend of successive signals having large amplitudes especially recognized in classical music, it will be understood that, by arranging the overall audio system so that signals having ordinary amplitudes are to be amplified by being applied with the abovesaid low voltage power supply, and that only the signals having large amplitudes are to be amplified by being applied with the above-mentioned high voltage power supply, it is possible to materialize a power amplifier concurrently having the advantages of a substantially expanded dynamic range and a high power efficiency.
In case, however, it is intended to use a power amplifier provided with two kinds of power supplies, one of which outputs a high voltage and the other outputs a low voltage as stated above so that the power supply voltages are switched between high and low levels in response to the signal amplitudes, it will be noted that, in view of the abovesaid fact that ordinary musical signals only occasionally contain signals of large amplitudes, the high voltage power supply would suffice only if it is capable of operating intermittently within a dynamic range in which the high voltage power supply can respond normally to such large amplitude signals. However, in case the amplifier system is arranged so that the abovesaid two kinds of power supplies are switched over between high and low voltage levels in response only to the amplitudes alone of the applied input signals, it will be noted that, in case a series of signals having large amplitudes, i.e. for example, tone burst signals which can not be included in ordinary musical signals, and a few kinds of electronic musical signals, are applied in succession to the amplifier for some reason or other, the high voltage power supply alone will be operated in succession, causing a large power to be consumed for an extended length of time. This gives rise to the necessity for setting the power capacity of the high voltage power supply and of the transistors of the output stage large enough, and this would undesirably lead to a high cost of manufacture of the entire amplifier system.